change: always apply Substitute map in alphabetic order

fix gosimple#8

slug.go

@@ -8,6 +8,7 @@ package slug
 import (
 	"bytes"
 	"regexp"
+	"sort"
 	"strings"
 
 	"github.com/rainycape/unidecode"
@@ -80,17 +81,24 @@ func MakeLang(s string, lang string) (slug string) {
 }
 
 // Substitute returns string with superseded all substrings from
-// provided substitution map.
+// provided substitution map. Substitution map will be applied in alphabetic
+// order. Many passes, on one substitution another one could apply.
 func Substitute(s string, sub map[string]string) (buf string) {
 	buf = s
-	for key, val := range sub {
-		buf = strings.Replace(buf, key, val, -1)
+	var keys []string
+	for k := range sub {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+
+	for _, key := range keys {
+		buf = strings.Replace(buf, key, sub[key], -1)
 	}
 	return
 }
 
 // SubstituteRune substitutes string chars with provided rune
-// substitution map.
+// substitution map. One pass.
 func SubstituteRune(s string, sub map[rune]string) string {
 	var buf bytes.Buffer
 	for _, c := range s {

slug_test.go

@@ -90,8 +90,8 @@ func TestSlugMakeUserSubstituteLang(t *testing.T) {
 		want string
 	}{
 		{map[string]string{"'": " "}, "en", "That's great", "that-s-great"},
-		{map[string]string{"&": "or"}, "en", "This & that", "this-or-that"}, // by default "&" => "and"
-		{map[string]string{"&": "or"}, "de", "This & that", "this-or-that"}, // by default "&" => "und"
+		{map[string]string{"&": "or"}, "en", "This & that", "this-or-that"},                   // by default "&" => "and"
+		{map[string]string{"&": "or"}, "de", "This & that", "this-or-that"},                   // by default "&" => "und"
 		{map[string]string{"&": "or", "@": "the"}, "de", "@ This & that", "the-this-or-that"}, // by default "&" => "und", "@" => "an"
 	}
 
@@ -143,7 +143,8 @@ func TestSubstituteLang(t *testing.T) {
 		want string
 	}{
 		{map[string]string{"o": "no"}, "o o o", "no no no"},
-		{map[string]string{"o": "no", "a": "or"}, "o a o", "no or no"},
+		{map[string]string{"o": "no", "a": "or"}, "o a o", "no nor no"},
+		{map[string]string{"a": "or", "o": "no"}, "o a o", "no nor no"},
 		{map[string]string{"'": " "}, "That's great", "That s great"},
 	}
 
@@ -165,6 +166,7 @@ func TestSubstituteRuneLang(t *testing.T) {
 	}{
 		{map[rune]string{'o': "no"}, "o o o", "no no no"},
 		{map[rune]string{'o': "no", 'a': "or"}, "o a o", "no or no"},
+		{map[rune]string{'a': "or", 'o': "no"}, "o a o", "no or no"},
 		{map[rune]string{'\'': " "}, "That's great", "That s great"},
 	}